This invention relates to a method for processing a watery substance, such as sludge or manure, wherein the watery substance is mixed with zeolites, the mixture of zeolites and watery substance is dried, the dried matter formed by the extraction of moisture from the watery substance is separated from the zeolites, and the zeolites are reused, as well as to an apparatus for carrying out such a method.
Such a method is known from Japanese patent application 55 013134, wherein such an amount of zeolites in granular form is added that the watery substance covers the surfaces of the granular zeolites thus functioning as carrier material, whereafter the drying of the sludge is accomplished by supplying hot air (hot air blast). From the so obtained mixture of zeolites and dried matter, the dried matter is separated, whereafter the zeolites can be used again. In such a direct drying method, intensive contact and hence a fast heat transmission occurs, but the off-gas processing can give rise to considerable problems and costs.
The object of the invention is for the processing of a watery substance, such as sludge or manure, to be carried out such that the amounts of off-gases are considerably reduced.
In accordance with the invention, this is achieved if in a mixer-drier the watery substance to be processed is mixed with zeolites of type A, which adsorb water from the watery substance while releasing adsorption heat which causes a further amount of moisture to evaporate from the watery substance, and the zeolites, after the separation of the dried matter, are regenerated through heating before being recycled to the mixer-drier for reuse. Through these measures, in the mixer-drier, the watery substance to be processed is brought into a direct intensive contact with the zeolites of type A, which, through their water selectivity, extract water from the watery substance through adsorption, by which mechanism adsorption heat is released which causes the temperature to rise, as a result of which, through evaporation of moisture, a further drying occurs. Discharging the evaporated moisture from the mixer-drier hardly, if at all, leads to off-gas problems. The amount to be added of zeolites of type A (NaA, KA, CaA), also referred to by the three-letter code LTA (Lindxc3xa9 Type A), is dependent on factors such as the desired solids content and the desired temperature of the mixture.
The substance thus dried to the desired value is subjected together with the zeolites to a heating, so that any residual moisture is removed and the zeolites are regenerated, i.e., by the supply of heat the previously adsorbed moisture is separated from the zeolites, and is subsequently discharged in vapor form, which entails few, if any, off-gas problems. The mixture left upon heating, of regenerated zeolites and dried matter residue can subsequently be separated, i.e., the zeolites can be recovered and can be recycled to the mixer-drier again for reuse. The separation can take place, for instance, by means of a screen, it being often preferred, in particular in the case of a fine powdered dried matter residue, that zeolites in a granulate form or a similar granular form are utilized. If, for instance, the dried matter residue to be obtained is a fibrous material, also zeolites in powder form can be of advantage.
With the method according to the invention, due to the water adsorbing action of the zeolites, not only the off-gas problem is reduced but so is the energy consumption. More particularly, with the method according to the invention, the following advantages are obtained:
minimum off-gas amounts and hence minimal costs for off-gas treatment;
high thermal efficiency owing to small gas flows to be heated;
short treatment time due to direct, intensive contact, and compact equipment;
no problems of caking onto the walls of the equipment;
no reflux of dry matter to the mixer-drier is needed;
high solids content at relatively low temperature;
fire-safe due to drying in an inert granule bed.
In a number of cases, for instance in the processing of manure, the dried substance may have commercial value. In those cases, in accordance with a further elaboration of the invention, it may be preferred that a part of the dried matter formed in the mixer-drier is extracted from the mixture of zeolites and dried matter before the mixture is heated for regenerating the zeolites.
Further, in the method according to the invention, advantageously, use can be made of the calorific value of the dried matter. If combustion air is supplied to the oven for at least partly combusting the dried matter in the mixture of zeolites and dried matter, then, depending on the organic fraction to be combusted in the dried matter, even an autothermic process can be obtained, so that the thermal efficiency of the process can be increased in that the supply of external heat can be dispensed with.
A further increase of the thermal efficiency can be obtained if the watery substance to be processed is first mechanically partly dewatered and/or preheated. This last can be done in a further advantageous manner in a pre-drier with vapor discharged from the mixer-drier, while condensate thereby formed is extracted from the mixer-drier.
The invention further relates to an apparatus for carrying out a method as contemplated in the foregoing. According to the invention, such an apparatus is characterized in that it comprises a mixer-drier with mixing means, a supply for watery substance to be processed, such as sludge or manure, a supply for zeolites, a discharge for vapor, and a discharge for a mixture, obtained by the mixing means, of zeolites and dried matter obtained by moisture extraction from the watery substance, to which last discharge transport means are connected which lead to a supply of an oven having, further, a discharge for vapor and discharge means for zeolites and dried matter residue, while separation means are present whence zeolites can be transported back to the mixer-drier.
To be able to separate substances that are of commercial value from the mixture before it is supplied to the oven, it may be provided that the transport means leading from the mixer-drier to the oven include a separation device capable of extracting at least a part of the dried matter from a mixture of zeolites and dried matter coming from the mixer-drier.
The thermal efficiency of the apparatus can be improved if the mixer-drier is preceded by a pre-drier having a supply for sludge to be processed, a supply for vapor coming from the mixer-drier, a discharge for condensate, and a discharge for pre-heated sludge.
For similar reasons, it may further be preferred that the oven comprises a supply for combustion air and a discharge for gases, while means are present for preheating the combustion air to be supplied, with the discharged gases. Thus, both the combustion heat of the organic fraction in the dried matter is fully utilized, and a part of the heat of the discharged air is utilized for preheating the combustion air.